Windows provide a significant design challenge for architects and builders. On the one hand, large, high transmission windows are desirable for providing an xe2x80x9copen airxe2x80x9d feeling to building occupants, significant solar or passive heat gain when the outdoor or ambient temperature is low and windows are desirable for allowing visible light into a building or structure to provide daylighting and thereby substantially reduce the need for electric lighting during the time when occupants are most often present. On the other hand, windows can allow in excessive amounts of solar heat when the outdoor or ambient temperature is high and air cooling or conditioning is already in use. This is especially true at certain times of the day and certain parts of a building when the solar radiation is shining directly on or through the windows.
An attempted solution to the problem is the use of window treatments like shades, drapes and blinds. However these are expensive, cumbersome and aesthetically undesirable solutions, particularly in large office buildings, hotel atria and public structures like airport terminals. Energy efficient windows based on thermochromic, photochromic, electrochromic and photoelectrochromic technologies have been proposed for providing variable transmission windows for use in buildings and structures. However, until now, none have shown the performance, durability, cost effectiveness and convenience to be used on a commercial basis.
This invention relates to energy efficient devices and windows that allow sunlight or solar radiation into a building or structure when the ambient temperature is low and substantially block solar radiation when the ambient temperature is high, especially when sunlight is directly on the window. This invention provides windows that allow passive solar heating and daylighting on colder days and still provide significant daylighting, while blocking solar heat build-up on warmer days, especially from sunlight shining directly on or through the windows of this invention. This invention also provides thermochromic devices such as variable transmission shutters for use as lenses or filters.
Ultimately, it is the outdoor or ambient temperature and the directness of the sun""s rays that determine the need for energy blocking character of windows. In a number of embodiments of this invention, the windows of this invention spontaneously change to provide energy blocking under the appropriate conditions of temperature and directness of sunlight without the control mechanisms and user intervention required by most alternate technologies under consideration for use as dimmable windows. Other embodiments of this invention provide windows that can be controlled by users or be controlled automatically by, for example, electronic control mechanisms, if so desired.
Windows and devices of the invention have residual light energy absorbing character such that when exposed to sunlight, (especially direct sunlight on warm or hot days), the temperature of at least a portion of the total window structure is raised significantly above the ambient, outdoor temperature. The windows and devices of the invention combine thermochromic character with this residual light energy absorbing character, juxtaposed in such a manner that there is an increase in temperature of the materials responsible for the thermochromic character when there is an increase in temperature due to sunlight exposure of the materials responsible for the residual light energy absorbing character. The thermochromic character is such that the total light energy absorbed by the window increases as the temperature of the materials responsible for the thermochromic character is increased from the ambient, outdoor temperature to temperatures above the ambient, outdoor temperature.
The residual light energy absorbing character is provided by static light energy absorbing materials and/or thermochromic materials that have some light energy absorbing character at ambient, outdoor temperatures. Preferably, any light energy absorbing character of the thermochromic materials at ambient outdoor, temperatures that contributes to the residual light energy absorbing character is due to the more colored form of the thermochromic materials that exists because of the thermal equilibrium between the less colored and more colored forms at outdoor, ambient temperatures or is due to the coloration of the less colored form and is not due to photochromic activity of the thermochromic materials. Preferably, the residual light energy absorbing character is such that the window is capable of absorbing about 5% or more and more preferably about 10% or more of the energy of solar irradiance incident on the window or device apart from any absorption changes caused by sunlight exposure. Preferably, the residual light energy absorbing character is such that there is a temperature increase in the materials responsible for the thermochromic character of at least 10xc2x0 C. and more preferably of at least 20xc2x0 C. above the ambient, outdoor temperature when the window or device is exposed to direct or full sunlight.
The thermochromic character can be provided by essentially any material or materials which change reversibly from absorbing less light energy to absorbing more light energy as the temperature of the material or materials is increased. It is preferred that the thermochromic character be provided by materials that have a smaller absorption at outdoor, ambient temperatures on warm and hot days and have an increase in absorption when the temperature of the materials responsible for the thermochromic character is increased at least 10xc2x0 C. It is preferred that the thermochromic character be provided by materials that have even less absorption at outdoor, ambient temperatures on cool and cold days and a less significant increase in absorption when the temperature of the window increases due to exposure to direct or full sunlight on cool and cold days.
The windows or devices of the invention optionally combine other characteristics like low emissivity, infrared light reflectance, barrier properties, protective overcoating, multipane construction and/or special gas fills to provide energy efficient windows.
Energy efficient windows and devices of the invention comprise one or more thermochromic layers which change from absorbing less light energy to absorbing more light energy as the temperature of the thermochromic layer(s) is increased. For many of the thermochromic layers used in the invention, this means a change from less colored to more colored as the temperature of the thermochromic layer(s) is increased.
Windows and devices of the invention comprise one or more substrates, (i.e. window pane, panel, light or sheet). The substrate may be a thermochromic layer or the substrate may have thermochromic layer(s) provided thereon. Windows of the invention may comprise two or more substrates spaced apart by spaces containing gas or vacuum.
Windows and devices of the invention optionally comprise a barrier to short wavelength light. The short wavelength light may be ultraviolet (UV) light. The short wavelength light may, optionally, include short wavelength visible (SWV) light. The barrier may absorb some or all of the UV and/or SWV light incident on the barrier layer. The barrier may be a substrate, a portion of a substrate, (e.g., the barrier may be in a polymeric layer adhering two sheets of glass together), or the barrier may be a layer provided on a substrate. The barrier, if present, is located between the sun and the thermochromic layer and serves to protect and/or modify the behavior of the thermochromic layer and possibly other layers present. The barrier can protect other layers, for example, from photodegradation by UV light and can modify the behavior of the thermochromic layer by suppressing some or all of the photochromic character of materials present which have both thermochromic and photochromic character. In many cases, the thermochromic materials will be incorporated into a polymeric material which includes an additive such as a UV stabilizer. While this stabilizer does not ordinarily provide the equivalent effect of a barrier layer, devices have been constructed without a barrier layer when a UV stabilizer is present in the thermochromic layer.
Windows and devices of the invention optionally comprise a protective overcoat. This overcoat, if present, serves to protect the thermochromic layer and optionally any other layer which may be present from, for example, physical abrasion, oxygen and environmental contaminants. The thermochromic layer is located between the sun and the protective overcoat, if it is present, e.g., a window pane of glass/thermochromic layer/protective overcoat may be oriented with the overcoat on the inside surface of the window structure.
Windows and devices of the invention optionally comprise one or more static light energy absorbing materials. These materials provide relatively constant light energy absorption, (i.e. absorption which is not significantly dependent on the temperature or photochemical processes of the light energy absorbing material). The static light energy absorbing material(s), if present, serves to provide residual light energy absorbing character and thus absorbs enough light energy during direct or full sunlight exposure to raise the temperature of at least a portion of the window above the ambient temperature surrounding the window. This helps to make the windows responsive to the directness of the sunlight. The static light energy absorbing materials may be contained in a separate layer, in the substrate, and/or any of the other layers present including the thermochromic layer as long as the absorbed energy is able to warm the themochromic material to a temperature at which the thermochromic material increases in sunlight absorption.
Windows and devices of the invention optionally comprise one or more low emissivity, (low-e), layers. The low-e layer(s) helps provide energy efficiency by its ability to reflect infrared, (IR), light and/or its ability to poorly emit or radiate IR light.
For the purposes of this invention, different types of light or electromagnetic radiation are defined, based on wavelengths of the radiation, as follows: